Solid electrolyte capacitors with a very large active capacitor surface area and therefore a small overall construction suitable for mobile communications electronics used are predominantly capacitors with a niobium or tantalum pentoxide barrier layer applied to a corresponding conductive substrate, utilizing the stability of these compounds (“valve metals”), the relatively high dielectric constants and the fact that the insulating pentoxide layer can be produced with a very uniform layer thickness by electrochemical means. The substrates used are metallic or conductive lower oxide (suboxide) precursors of the corresponding pentoxides. The substrate, which simultaneously forms a capacitor electrode (anode) comprises a highly porous, sponge-like structure which is produced by sintering extremely fine-particle primary structures or secondary structures which are already in sponge-like form. The surface of the substrate structure is electrolytically oxidized (“formed”) to produce the pentoxide, with the thickness of the pentoxide layer being determined by the maximum voltage of the electrolytic oxidation (“forming voltage”). The counterelectrode is produced by impregnating the sponge-like structure with manganese nitrate, which is thermally converted into manganese dioxide, or with a liquid precursor of a polymer electrolyte followed by polymerization. The electrical contacts to the electrodes are produced on one side by a tantalum or niobium wire which is sintered in during production of the substrate structure and on the other side by the metallic capacitor sheath, which is insulated with respect to the wire.
The capacitance C of a capacitor is calculated using the following formula:C=(F·ε)/(d·VF)where F denotes the capacitor surface area, ε the dielectric constant, d the thickness of the insulator layer per V of forming voltage and VF the forming voltage. Since the dielectric constant ε is 27.6 or 41 for tantalum pentoxide or niobium pentoxide, respectively, but the growth in the layer thickness per volt of forming voltage d is 16.6 or 25 Å/V, both pentoxides have an almost identical quotient ε/d=1.64 or 1.69, respectively. Capacitors based on both pentoxides, with the same geometry of the anode structures, therefore have the same capacitance. Trivial differences in details concerning specific weight-related capacitances result from the different densities of Nb, NbOx (0.7<x<1.3; in particular 0.95<x<1.1) and Ta. Anode structures made from Nb and NbOx therefore have the advantage of saving weight when used, for example, in mobile telephones, in which every gram of weight saving is a priority. With regard to cost aspects, NbOx is more favourable than Nb, since some of the volume of the anode structure is provided by oxygen.
The niobium suboxide powders are produced using the standard metallurgical reaction and alloying processes, according to which a mean oxide content is produced by exposing niobium pentoxide and niobium metal, in the presence of hydrogen, to a temperature at which an oxygen concentration balancing takes place, cf. for example WO 00/15555 A1:2Nb2O5+3Nb→5NbO  (1)
The process therefore comprises the use of a high-purity commercially available niobium pentoxide and mixing it with high-purity niobium metal, both in powder form corresponding to the stoichiometric proportions and treating them for several hours at a temperature of from 800 to 1600° C. in a hydrogen-containing atmosphere, which should preferably contain up to 10% of hydrogen. It is preferable for both the pentoxide and the metal to have primary particle sizes which, after the oxygen balancing has taken place, correspond to the desired primary particle size of less than or slightly over 1 μm (smallest) cross-sectional dimension.
In this process, crucibles made from niobium or tantalum which have been filled with a mixture of niobium pentoxide and niobium metal powders are heated to the reaction temperature in a furnace under a hydrogen-containing atmosphere. The niobium metal required for the oxygen exchange with niobium pentoxide is preferably produced by reduction of high-purity niobium pentoxide to form the metal.
This can be effected aluminothermically by igniting an Nb2O5/Al mixture and washing out the aluminium oxide which is formed and then purifying the niobium metal ingot by means of electron beams. The niobium metal ingot obtained after reduction and electron beam melting can be embrittled using hydrogen in a known way and milled, producing plateletlike powders.
According to a preferred process for producing the niobium metal in accordance with WO 00/67936 A1, the high-purity niobium pentoxide powder is firstly reduced by means of hydrogen at 1000 to 1600° C. to form the niobium dioxide of approximately the formula NbO2, and is then reduced to the metal using magnesium vapour at 750 to 1100° C. Magnesium oxide which is formed in the process is washed out by means of acids. The latter process is preferred in particular on account of its considerably lower energy demand, on account of the fact that the primary particle size of the niobium pentoxide is in principle maintained and that there is a lower risk of contamination with substances which are harmful to the capacitor properties.
One drawback of the reaction in accordance with reaction Equation (1) is that the volumetric shrinkage of the niobium pentoxide during the transition to the niobium suboxide amounts to approx. 50%, which causes a very loose crystal microstructure of the suboxide which can only be densified by conditioning with a risk of crystal defects being incorporated, and therefore may ultimately have an adverse effect on the capacitor properties. The poor crystal quality of the suboxide is evidently also a reason for its inadequate flow properties.
Good flow properties of the capacitor powders represent a significant process parameter in the production of the capacitors, since the powders are pressed by means of automatic high-speed pressers which are supplied with the powder to be pressed via storage containers. Good flow properties represent a precondition for a defined quantity of powder to flow into the press mould with an accuracy which satisfies modem-day requirements, for example of +/−0.5 mg.